Permanent deformations in the solid matrix can be caused by many field scenarios, such as high injection rates. A pressure differential in the field can create geomechanical loading of large magnitude that may increase stress from an elastic regime to a plastic regime. Simple geomechanical models based on linear elasticity are insufficient in predicting these types of effects. To accurately predict rock formation damage and failure responses, nonlinear analyses based on geomaterial plasticity models should be included in modeling frameworks through rigorous coupling with reservoir flow simulators.
In this work we integrate an implementation of the Drucker-Prager plasticity model into the parallel compositional reservoir simulator, IPARS (Integrated Parallel Accurate Reservoir Simulator). Fluid flow is formulated on general distorted hexahedral grids using the multipoint flux mixed finite element method. The mechanics and flow systems are solved separately and coupled using a fixed-stress iterative coupling algorithm. This allows multiple flow models to be used with nonlinear mechanics without modification, and allows each type of physics to employ the best preconditioner for its linear systems. The fixed-stress iteration converges to the fully coupled solution on each time step.
With these components in place, we conduct a study on wellbore stability using different flow and geomaterial models. We demonstrate the capabilities of our integrated simulator in predicting near-wellbore plastic strain development and its effect on multiphase component concentrations. Our simulations run efficiently in parallel using MPI on high performance computing platforms up to hundreds or thousands of processors. The results of the simulations are useful in predicting wellbore failure.
Our integrated simulator has several distinctive features. The use of general hexahedral finite element grids is particularly well-suited to handle domain specific applications such as near-wellbore studies. The multipoint flux scheme is an accurate and convergent method, it is locally conservative, and its linear systems are efficiently solved with multigrid methods. The use of a fixed-stress iterative coupling scheme is novel for coupling nonlinear mechanics with compositional fluid flow. Finally, to achieve fast convergence rates for solving nonlinear solid mechanics problems, a material integrator has been consistently formulated to give quadratic convergence rates.